Superabsorbent polymers have been developed in recent years that are capable of absorbing many times their own weight of liquid. These polymers, which are also known as water insoluble hydrogels, have been used to increase the absorbency of sanitary products such as diapers and sanitary napkins. Superabsorbent polymers are often provided in the form of particulate powders, granules, or fibers that are distributed throughout absorbent cellulosic products to increase the absorbency of the product. Superabsorbent particles are described, for example, in U.S. Pat. No. 4,160,059; U.S. Pat. No. 4,676,784; U.S. Pat. No. 4,673,402; U.S. Pat. No.5,002,814; and U.S. Pat. No. 5,057,166. Products such as diapers that incorporate absorbent hydrogels are shown in U.S. Pat. No. 3,669,103 and U.S. Pat. No. 3,670,731. Other types of particles, that perform specific desired functions in end products, are also sometimes added to fibrous webs. These particles include, for example, antimicrobials, fire retardants, zeolites, odor absorbents, and the like.
One problem with the use of particles to impart properties to a fibrous web is that the particulate material can be physically dislodged from the fibers of an absorbent product. The physical separation of particles from fibers usually takes place during mechanical handling and transportation of the particle-containing fibrous web. This separation leads to undesirable and deleterious effects on the end product. For example, separation of superabsorbent particles from its substrate usually reduces the absorbency of the product and diminishes the effectiveness of the superabsorbent material. This problem was addressed in European Patent Application 442 185 A1, which discloses use of a polyaluminum chloride binder to bind an absorbent polymer to a fibrous substrate. The polyaluminum binder, however, suffers from the drawback of being an inorganic product that is not readily biodegradable. Moreover, that European patent does not offer any guidance for selecting binders other than polyaluminum chloride that would be useful in binding absorbent particles.
A method of immobilizing superabsorbents is disclosed in U.S. Pat. No. 4,410,571 in which a water swellable absorbent polymer is converted to a non-particulate immobilized confluent layer. Polymer particles are converted to a coated film by plasticizing them in a polyhydroxy organic compound such as glycerol, ethylene glycol, or propylene glycol. The superabsorbent assumes a non-particulate immobilized form that can be foamed onto a substrate. The individual particulate identity of the superabsorbent polymer is lost in this process. The confluent nature of the superabsorbent material can also result in gel blocking, in which absorption is diminished as the water swollen polymers block liquid passage through the film layer.
U.S. Pat. No. 4,412,036 and U.S. Pat. No. 4,467,012 disclose absorbent laminates in which a hydrolyzed starch polyacrylonitrile graft copolymer and glycerol mixture is laminated between two tissue layers. The tissue layers are laminated to each other by applying external heat and pressure. The reaction conditions form covalent bonds between the tissue layers that firmly adhere the tissue layers to one another.
Numerous other patents have described methods of applying binders to fibrous webs. Examples include U.S. Pat. No. 2,757,150; U.S. Pat. No. 4,584,357; and U.S. Pat. No. 4,600,462. Such binders are not described as being useful in binding particulates, such as superabsorbent particles, to fibers.
Yet other patents disclose crosslinking agents such as polycarboxylic acids that form covalent intrafiber bonds within individual cellulose fibers, as in European Patent Application 440 472 A1; European Patent Application 427 317 A2; European Patent Application 427 316 A2; and European Patent Application 429 112 A2. The covalent intrafiber bonds are formed at elevated temperatures and increase the bulk of cellulose fibers treated with the crosslinker by forming intrafiber ester crosslinks. The covalent intrafiber bonds produce a fiber product that when airlaid into a fibrous web yields a web that is more difficult to compress to conventional pulp sheet densities than an untreated sheet. Covalent crosslink bonds may also form between the fibers and particles, and thus occupy functional groups of the fibers that would otherwise be available for absorption, hence absorption efficiency is decreased.
A particular disadvantage, for some applications, of forming covalent ester intrafiber crosslinks is that the resulting crosslinked, stiffened fiber product resists densification. Energy requirements for making densified absorbent products are therefore increased because very high compression pressures must be applied to density the absorbent product.
Some of the foregoing and other problems have been overcome by technology of the parent and related applications which provide more readily densified fibrous webs that are made of fibers with hydrogen bonding functional sites, and binders, less volatile than water, that have a functional group capable of forming a hydrogen bond with the fibers, and another or the same functional group that is also capable of forming a hydrogen bond or a coordinate covalent bond with particles. The binders of the parent and related applications are either polymeric or non-polymeric. The polymeric binders may be selected from the polyglycols [especially poly(propyleneglycol)], a polycarboxylic acid, a polycarboxylate, a poly(lactone) polyol such as diols, a polyamide, a polyamine, a polysulfonic acid, a polysulfonate and the like, and combinations thereof Specific listed examples of some of these binders, are as follows: polyglycols including polypropylene glycol (PPG) and polyethylene glycol (PEG); poly(lactone) diols including poly(caprolactone) diol; polycarboxylic acid including polyacrylic acid (PAA); polyamides including polyacrylamide or polypeptides; polyamines including polyethylenimine and polyvinylpyridine; polysulfonic acids or polysulfonates including poly(sodium4-styrenesulfonate) or poly(2-acrylamido-methyl-1-propanesulfonic acid); and copolymers thereof (for example a polypropylene glycol/polyethylene glycol copolymer).
The non-polymeric binder noted above is disclosed as having a volatility less than water and has at least one functional group that is capable of forming a hydrogen bond or coordinate covalent bond with the particles, and at least one functional group that is capable of forming hydrogen bonds with the cellulose fibers. The non-polymeric binder is described as an organic binder including a functional group selected from a carboxyl (for example, carboxylic acids), a carboxylate, a carbonyl (for example, aldehydes), a sulfonic acid, a sulfonate, a phosphoric acid, a phosphate, a hydroxyl (for example, an alcohol or polyol), an amide, amine, and the like, and combinations thereof (for example, amino acid or hydroxy acid), wherein there are at least two functionalities on the molecule selected from this group, and the two functionalities are the same or different. Polyols, polyamines (a non-polymeric organic binder with more than one amine group), polyamides (a non-polymeric organic binder with more than one amide group), polycarboxylic acids (a non-polymeric organic binder with more than one carboxylic acid functionality), amino alcohols, and hydroxy acids are listed as examples of such binders. These binders have functional groups that are capable of forming the specified bonds with the particles and fibers.
The amount of binder present is described as depending on a number of factors, including the nature of the binder and particles, and whether the particles are immediately added to the fibers or after a period of time. Hence, one skilled in the art will realize that the amount of binder suitable and particularly useful for a particular application will vary. However, it is disclosed that the binder may suitably be present in an amount of from about 1 to 80 percent of the total weight of the fibrous material An especially suitable disclosed range of binder is 1 to 40 percent by weight, or 1 to 25 percent by weight of the fibrous material. The particles bound by the binder (via hydrogen/coordinate covalent bonds) may suitably be present in an amount of 0.05 to 80 percent, preferably 1 to 80 percent or 3 to 80 percent, or more than 3 percent by weight of the total weight of the fibrous material and the particles.
A particularly suitable range of particles disclosed in the related applications is 3 to 40 percent by weight of the fibrous material and particles. A preferred weight ratio of particle to binder is 8:1 to 50:1. Suitable particles are superabsorbent polymer particles such as a starch graft polyacrylate hydrogel fines or larger size particles such as granules, which form hydrogen bonds with the binder. The related applications teach that the binder also forms hydrogen bonds with the hydroxyl groups of the cellulose, thereby securely attaching the superabsorbent particles to the fibers.
In some instances, according to the related applications, the binder is associated with the fibers as a solid (for example, a dry powder or a dried liquid), and the fibers contain at least 7 percent water by weight when the binding step is performed. This level of moisture in the fibers provides sufficient mobility of reactants to allow the particles and fibers to bind well to each other. When a liquid binder is used (for example, glycerin or a solution of glycine powder), the fibers suitably contain at least about 0.05 percent water by weight.
Moreover, it is discussed that the capacity for activation or reactivation allows the binder to be applied to the fibers, which are then shipped to distribution points with the binder in an inactive form. The binder is then activated at the distribution point (for example, a customer's facility) where particles are added to the fibers and bound thereto. As used therein, binder "activation" includes both activation of previously inactive binders (such as solid binders in the absence of liquid) or reactivation of previously active binders (such as a liquid binder that has been dried).
Of the useful binders, a significant proportion are acidic so that the pH of a liquid absorbent product may be adjusted when the product is wetted with, for example, synthetic urine. It is generally preferred, however, for health reasons that the pH of disposable diapers, a commercially important product using particles (especially superabsorbent particles), be maintained at above about pH 4 and as close to a neutral pH as possible.
Despite the availability of binders using a hydrogen bonding and/or coordinate covalent bonding mechanism for binding particles to fibers and fibers to fibers, there exists a need for binders that more strongly attach the particles to the fibers. This need is especially critical when the fiber-particle combination must undergo intensive mechanical handling, as in transportation, storage and reprocessing of the combined material Under such intensive handling, it has been found that particles bound by a binder through hydrogen bonding or coordinate covalent bonds to fibers may become dislodged and migrate from a position where their presence is required.